MOSCOW -- Prime Minister Vladimir Putin pledged Tuesday to drag "from the bottom of the sewers" those behind the deadly attack on the Moscow subway system, but some Russians began to challenge his government for failing to prevent the suicide bombings despite signs that Islamist rebels had been preparing to strike.

As the nation observed a day of mourning and nervous Muscovites returned to the subway, public debate shifted toward how the Kremlin should respond to Monday's bombings and whether Russia's powerful security services could have stopped the attack, which killed 39 people and injured more than 70.

Internet users flooded President Dmitry Medvedev's blog with notes of sympathy for families of the victims but also blunt criticism of law-enforcement agencies. Some accused police of being more interested in collecting bribes than tracking down terrorists. Others asked why modern equipment to detect explosives had not been installed as promised after the last subway bombing in Moscow nearly six years ago.

"Was it just talk and forget, as always?" one commenter wrote. "The impression is that today's tragedy on the Moscow subway is the direct result of the 'efficient' spending of budget funds by the senior ranks of the police."

Gennady Gudkov, a member of Putin's ruling party who is on the security committee in the lower house of parliament, said the criticism was natural because the attack was "the direct result of mistakes and miscalculations by the security services" and "everybody believes the state should protect them."

"The problem of terrorism has been unsolved all these years. It's a legitimate question for Prime Minister Putin and President Medvedev," he said, arguing that the Federal Security Service, or FSB, the main successor to the Soviet-era KGB, had failed to develop intelligence sources in the insurgency in the North Caucasus that has been linked to the bombings.

Andrei Soldatov, an investigative reporter who runs a Web site that covers Russian security agencies, said the FSB ignored early signs the rebels were shifting tactics and preparing to resume attacks in Moscow, including a suicide bombing in the southern city of Vladikavkaz in November 2008 and an announcement last year that a brigade that staged attacks across Russia from 2002 to 2006 had been revived.

"The FSB is still inclined toward a shoot-to-kill policy in the North Caucasus," he said. "This approach works against large groups of militants in the forests but not against would-be suicide bombers."

Soldatov said the warnings should have led the FSB to redouble efforts to develop sources and win over the public, but it didn't. "They simply love their brutal tactics," he said. "Nothing was proposed to deal with the Internet, for example."

As the search for accomplices in the attack continued, police were trying to determine whether the two female bombers might have been trained by Alexander Tikhomirov, an Islamist preacher killed by security forces this month. Local news media cited unnamed sources as saying that Tikhomirov may have recruited 30 suicide attackers before his death.

The Kremlin said it planned to propose new legislation to fight terrorism but provided no specifics, while some lawmakers proposed bringing back the death penalty.

Opposition leaders predicted that Putin would use the bombings to justify a crackdown on dissent, noting that he consolidated power after a rash of attacks in the first part of the 2000s. "There will be more censorship, political spying. There will be more riot police dispersing opposition rallies and protests," Boris Nemtsov, a former deputy prime minister, said in an online editorial.

But Vladimir Ryzhkov, another opposition leader, said the public would not accept a wave of repression because the bombings had undermined Putin's claim to have restored stability to Russia, while a severe recession has damaged his record of delivering economic growth.

"I think the regime is much weaker now than a year or two years ago because the public is raising more questions, about the economy, about the efficiency of the government, and now about security," he said.

Ryzhkov added that he hoped Medvedev would use the crisis to investigate and take greater control of the security services, which some argue wield more power in Russia than any other institution. But such a move is unlikely because the FSB is protected by Putin, who once led the agency and remains the nation's senior leader.

In televised remarks, Putin showed no sign of tempering the government's sometimes brutal efforts to crush the insurgency. "We know that they are lying low," he said of those involved in the subway attack. "But it is already a matter of honor for law-enforcement bodies to drag them from the bottom of the sewers and into the daylight."

Medvedev, Putin's handpicked successor as president, has struck a tough tone as well. But he added Tuesday that the government must also improve social and economic conditions in the North Caucasus, signaling that he intends to press ahead with the moderate policies he has promoted in the region to draw support away from the insurgents.

"This job is even harder than looking for and destroying terrorists," he said. "But we will do it anyway, as well as establish order by using forcible methods."